Recently, digital signage is a target of attention. “Digital signage” is a form of information communication medium using digital technologies. Digital signage is mainly installed in places where many people gather outside general houses (e.g., stations and airports, shopping malls, public facilities, etc.) to display advertisements, news and other information in the form of images such as still images, moving images, subtitles and the like. As specific devices which realize digital signage, liquid crystal display devices are preferably used.
A liquid crystal display device includes a liquid crystal panel, which includes a pair of light-transmissive substrates and liquid crystal molecules enclosed therebetween, and a backlight unit located on a rear surface of the liquid crystal panel. With the liquid crystal display device, light emitted by the backlight unit is directed from the rear side of the liquid crystal panel, and thus an image displayed on the liquid crystal panel is made visually recognizable. When such a liquid crystal display device is used for digital signage, images such as still images, moving images, subtitles and the like can be displayed with bright colors and a high degree of freedom of expression. This significantly improves the ability of expression of digital signage as an information communication medium.
However, when a liquid crystal display device is used for digital signage, there is a problem that the initial cost at the time of installment and also the running cost are high. A main reason for the high running cost is a high power consumption of a light source built in the backlight unit described above. Recently, the size of the display screen of a liquid crystal display device is significantly increased. In a liquid crystal display device including such a large display screen, the power consumption of the light source built in the backlight unit is likely to be increased. Recently, a light emitting diode (LED), which consumes a relatively small amount of power, has been used as a light source of the backlight unit. Nonetheless, the power consumption needs to be further reduced, considering that the liquid crystal display device is used for a long term.
Thus, a liquid crystal display device which generates power by use of a solar panel and uses the generated power to drive the light source and thus decreases the power consumption has been proposed. However, when this liquid crystal display device is used for digital signage, there is a problem that the installment cost is high because the solar panel costs relatively high. In addition, the solar panel does not necessarily have a long life and thus needs to be replaced periodically. This causes another problem that the running cost results in being high.
Aside from a liquid crystal display device used for digital signage, it has been proposed that a display device for displaying still images such as signs or the like collects natural light such as sunlight or the like by a use of light collecting section and transports the light to a display panel by an optical fiber or the like, so that the light is used as backlight (see, for example, Patent Document 1). In the case where a liquid crystal panel is used as the display panel of the display device, the collected light can be used as it is as backlight of the liquid crystal panel. This can reduce the power consumption and also can extend the life of the liquid crystal panel. As a result, the running cost can be significantly decreased. With such a structure, the installment cost of the display device can be significantly decreased as compared with the case where the above-described liquid crystal display device including the solar panel is used.